In vivo Micropuncture Studies: 1) Elevation of bicarbonate transport in the rat distal convoluted tubule utilizing in vivo microperfusion techniques; 2) The effect of PTH on bicarbonate transport in the proximal convoluted tubule; 3) Direct determination of disequilibrium pH in the distal tubule under various physiological conditions; and 4) Quantitative assessment of the role of bicarbonate "back-leak" in determining net volume reabsorption across in vivo microperfused proximal tubules under various physiological conditions. In Vitro Microperfusion Studies: 1) Quantitative comparison of the capacity of the cortical versus medullary collecting duct to reabsorb or secrete bicarbonate; 2) Examine the in vitro factors which regulate bicarbonate transport in collecting duct segments with special emphasis on pCO2; amiloride; ambient (HCO3); cAMP and PGE2; 3) Determine whether two populations of proximal tubules exist in newborn rabbits. If they do not, then design a series of experiments to find factors responsible for development of a second population of proximal tubules; 4) Characterize further the nature of active chloride (? secondary active) transport across thick limbs of Henle by use of Na and K free solutions and by use of compounds which regulate intracellular cAMP content; and 5) Determine pCO2 permeability across the collecting duct segments.